Photosensitivity is sensitivity to flickering or intermittent
light stimulation and visual patterns. It affects approximately one in
four thousand people. A number of people have this sensitivity but have
not yet had a seizure and therefore have not been diagnosed with the condition
of photosensitive epilepsy. The most common trigger for photosensitive
epilepsy in Europe is the domestic television set. Almost fifty percent
of patients are sensitive to the 50Hz flicker of television, and some seventy
five percent of patients are sensitive to the 25 Hz flicker from the line
raster which can be observed with close viewing. The onset of photosensitive
epilepsy in an individual occurs typically around the time of puberty;
in the age group 7 to 20 years the condition is five times as common as
in the general population. Three quarters of patients remain photosensitive
for life.

In response to a Pot Noodles advert in 1993 which induced
photosensitive epileptic seizures in 3 people, the ITC introduced its Guidelines
for Flashing Images and Regular Patterns. The sequences to be avoided sound
relatively simple - repetitive bright or red flashes and spatial patterns
- but the details are complicated.

In December 1997 a children's Pokemon cartoon episode
in Japan produced 685 admissions to hospital. 560 cases were shown to have
had proved seizures, triggered by four seconds of alternating saturated
red and blue light used in the programme. Of those patients, 76 percent
had no previous history of seizures. The Guidelines have since been updated.

Professor Graham Harding, an expert on Photosensitive
Epilepsy who assisted with the drafting of the Guidelines, also collaborated
in developing the Harding FPA.

The article on this affliction is interesting. It is
true that television flickers at the frame rate. The screen phosphor storage
time (the amount of time a phosphor dot on the screen stays on after the
electron beam passes it) is measured in milliseconds. Hence, the entire
screen will flicker as the author stated. I also know someone with photosensitive
epilepsy. For him the flicker rate is different - he can look at televisiona
and even works with computer monitors. However, sunlight passing through
leafless branches of trees while in a moving vehicle will trigger an attack
for him. He can never drive, and his wife must drive him everywhere.

But, there is help for those sensitive to television
screens (although still not a cheap solution.) Newer LCD television screens
create an image without using phosphor scanning. Hence, there is no 50Hz
flicker. Although LCD panels flicker at the DC to AC inverter rate crated
by the florescent backlight drive circuit, this frequency is up above 20KHz
and therefore not an issue. The photoreceptors in the eye cannot respond
that fast, and therefore do not send any pulsed neural signals to the brain
to trigger epilepsy.

One way to test a CRT for flicker, is to use your peripheral
vision. These photoreceptors respond much faster than pulsed light near
the center of the eye. Look off to the side of any CRT screen at about
a 45 degree angle, and see if any flicker is perceptable. Even a computer
monitor running at 75Hz can be faintly observed to flicker using this test.
An LCD panel will not flicker, because of a much higher time constant for
each pixel.

Ted Twietmeyer

Comment

Ellen J. Botelho, OT

2-18-4

This was a great, informative article, but it sadly missed
a big problem we epileptics face everyday: strobe lights. They are now
on emergency vehicles, school busses, and in some public buildings fire
alarm systems. My seizures are gone now, but I still have an almost instinctive
feeling of dread whenever I drive behind a school bus or police car. I
can't help but think: what if I weren't so lucky? What if every time I
saw a strobe light, it triggered a seizure? Why didn't someone stop this
trend, before they were all over our society?

So much for the Americans With Disabilities Act. The
use of strobe lights all over our society is harmful to the health of epileptics,
and no one seems to care. Many of us are imprisoned in our homes, because
venturing out into society can be harmful to us.